going sliding on the saar!

what do these lines mean ?

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This means that Franz wasted his time in enjoying and now when he came to know that it would be their last lesson he felt guilty that he should not have wasted his tims

This phrase means simply that Franz is recalling and remembering all those activities such as collections birds eggs ,playing with slide on Saar .He never had his interest on studies .But soon he realized how wrong he has done, he is repenting now on his deeds. Thanks 😊